1. Technical Field
This disclosure relates to semiconductor fabrication tools and more particularly, to an improved system and method for generating patterns on reticles used in semiconductor fabrication processes.
2. Description of the Related Art
Semiconductor fabrication processes typically include photolithographic processing to pattern areas of a surface of a semiconductor device. The semiconductor fabrication process typically includes applying a photoresist material to the surface of the semiconductor device. The photoresist is patterned by exposing the photoresist to light, typically ultraviolet light, to crosslink the resist material (negative resist). This cross linking prevents a reaction with a developer which develops away areas of the photoresist which were not crosslinked by the exposure to the UV light. Other types of photoresists have chains broken by exposure (positive resist) to ultraviolet light.
Photoresists are patterned using a photomask. The photomask functions as a shield to prevent light form passing through it in predetermined areas during photolithography. The photomask typically provides a black or highly absorbent layer of material, usually chromium or a chromium alloy, patterned in accordance with the patterning design to be projected onto the photoresist. The absorbent layer is formed on a substrate, which may include a glass or quartz material. Other techniques are used which may include electrons and electron beam masks, scattering masks and/or stencil masks, for example, scattering with angular limitation in projection electron beam lithography (SCALPEL).
With decreasing feature sizes of semiconductor components, masks are increasingly more difficult to fabricate and inspect. It is known that advanced semiconductor processing is very sensitive to image quality provided by masks. The defect fabrication capability for reticles is limited to a certain minimum feature size. This minimum feature size typically depends on the process and fabrication tools used to provide the pattern on the reticle.
Reticles may be written by laser pattern generators or electron beam pattern generators. Since reticles typically include a multitude of features below a micron in size. Fabrication is performed using automated devices. Referring to FIG. 1, a reticle fabrication device 10 is shown. Device 10 includes a stage 14 for positioning a mask or reticle 16 to be fabricated. An energy source 18 provides a laser beam or an electron beam for writing a pattern on mask 16 with a predetermined intensity of light or electrons. Mask 16 is preferably guided by stage 14 according to a computer generated image of the pattern to be written on mask 16.
Both laser and electron beam pattern generators have the capability for complex reticle patterns, including those with narrow geometries, dense optical proximity correction (OPC) and phase shift masks (PSM). OPC helps compensate for lost light to ensure that the precise patterns are formed on a semiconductor wafer. For example, without OPC, a rectangle can end up looking like an oval on the wafer because light tends to round on the edges. OPC corrects this by adding tiny serifs (lines) to the corner to ensure that the corners are not rounded or moving a feature edge so wafer features are sized more accurately. Phase shift masks alter the phase of light passing through the photomask, and permit improved depth of focus and resolution on the wafer. Phase-shift helps reduce distortion on line resolution of wafer surface irregularities.
Although laser pattern generator provide higher reticle throughput, lower cost and better placement accuracy, laser pattern generators produce large corner rounding. Referring to FIG. 2, a circular laser/electron beam spot 30 is shown for writing a pattern on for a reticle on a mask blank 32. Mask blank 32 includes a resist layer 33 formed on a blank mask 32. A pattern 34 is formed by exposing portions of resist to light or electrons. The pattern is created by applying laser/electron spot 30 thereon to expose resist 33. Typically, blank mask 32 includes an energy absorbent material, such as, chromium, molybdenum or their alloys, or metal oxides on a glass or quartz substrate. After exposure resist 33 is developed and exposed portions of energy absorbent material on blank mask 32 is etched away. As laser/electron spot 30 approaches a corner 38, resist 33 cannot be exposed in corner 38 as a result of the geometry of laser/electron spot 30. This is referred to as corner rounding. The large corner radius is related to the beam diameter as approximately equal to 1.17.times.beam diameter for laser beams. State of the art tools are capable of corner rounding as low as 300 nm.
Conventional solutions to large corner rounding include employing serifs, hammerheads and other types of add-on structures. The addition of these structures adds to the complexity of the reticle pattern, increases data volume for storing the reticle design and makes the reticle pattern more difficult to inspect due to the add-on features.
Therefore, a need exists for a system and method which reduces corner rounding in reticle fabrication processes. A further need exists for a system and method which reduces the need for add-on structures in reticle fabrication processes.